In semiconductor devices including a DRAM (Dynamic Random Access Memory), defective cells that do not operate properly are replaced with redundancy cells to relieve defective addresses. Usually, fuse elements are utilized to store the defective addresses. Laser beams are irradiated to the fuse elements so as to disconnect them irreversibly, so that the defective addresses are stored. The usual fuse elements can store information in a nonvolatile manner by changed from the conductive state to the isolated state.
Meanwhile, antifuse elements have attracted attention in recent years (see U.S. Pat. Nos. 6,902,958 and 6,700,176, and U.S. Patent Application Publication No. 2005/0258482). The antifuse elements store information, as opposed to the usual fuse elements, by changed from the isolated state to the conductive state. The configuration of the antifuse elements is almost the same as that of depletion MOS transistors. When the gate insulating film is subjected to breakdown by a high voltage applied between the gate electrode and the electrode common to the source and drain, the antifuse element is changed from the isolated state to the conductive state.
Because the antifuse element has the same configuration as the depletion MOS transistor, its occupied area is smaller than the usual fuse element and the passivation film is not broken by the laser irradiation.
However, the antifuse element has a problem of a large variation in resistance in the conductive state. Reasons why the resistance is varied in the conductive state are explained below.
FIG. 9 is a schematic diagram of a configuration of a conventional antifuse element.
As shown in FIG. 9, a generally used antifuse element includes a gate electrode 12, a source region 14, and a drain region 16. The source region 14 and the drain region 16 are shorted by the wirings. In the initial state, the gate electrode 12 is isolated from a channel region 20 by a gate insulating film 18. The gate electrode 12 is thus isolated from the source region 14 and the drain region 16. When a breakdown region 18a is formed in the gate insulating film 18 by applying a high voltage to the gate electrode 12, the gate electrode 12, the source region 14, and the drain region 16 are short-circuited via the depletion channel region 20.
Thus, by detecting whether current flows between a terminal D connected to the gate electrode 12 and a terminal E connected to the source region 14 and the drain region 16, whether the breakdown region 18a is formed in the gate insulating film 18 is determined.
FIG. 10 is an equivalent circuit diagram of the antifuse element in the breakdown state.
As shown in FIG. 10, if the antifuse element is subjected to breakdown, a resistance component Rg for the gate electrode 12 and the breakdown region 18a is connected to a parallel circuit of a channel resistance component Rs on the source region 14 side and a channel resistance component Rd on the drain region 16 side between the terminals D and E. The resistance components Rs and Rd vary depending on the position of the breakdown region 18a formed. The position of the breakdown region 18a formed depends on a predetermined probability distribution. The breakdown region 18a can be formed in the vicinity of the source region 14 or the drain region 16, or can be formed at the substantial intermediate position between the source region 14 and the drain region 16.
When the breakdown region 18a is formed in the vicinity of the source region 14 or the drain region 16, one of the resistance components Rs and Rd is reduced significantly. The resistance between the terminals D and E is thus relatively small. When the breakdown region 18a is formed at the substantial intermediate position between the source region 14 and the drain region 16, the resistance components Rs and Rd are increased, resulting in relatively large resistance between the terminals D and E.
When Rs=Rd, that is, when the breakdown region 18a is formed at the intermediate position, the resistance Rde between the terminals D and E is given by the following formula.Rde=Rg+Rs·Rd/(Rs+Rd)=Rg+Rd/2
In contrast, when Rs>>Rd, that is, when the breakdown region 18a is formed at either of the ends (e.g., in the vicinity of the drain region 16), Rd≈0. The resistance Rde between the terminals D and E is given by the following formula.Rde=Rg+Rs·Rd/(Rs+Rd)≈Rg 
A normal sheet resistance is a hundred and several tens Ω/□ on a gate resistance layer and a few KΩ/□ to a several hundred MΩ/□ on a depletion channel resistive layer. The resistance Rde depends substantially on the depletion channel resistive layer.
Even if the antifuse element is subjected to breakdown in the substantially same voltage conditions, the resistance between the terminals D and E is inevitably varied greatly. Such a variation makes it difficult for thresholds to be set when it is determined whether the breakdown region 18a is formed. Sometimes that may cause wrong determination. The resistance between the terminals D and E may be relatively large even after breakdown. The detection sensitivity needs to be set high to some extent. The time required for determination is extended, which prevents high speed operation of semiconductor devices.